1. Field of the Invention
This invention relates in general to offshore drilling equipment, and in particular to a locking means that can be used for locking an inner tubular member within an outer tubular member, such as a casing hanger within a wellhead or a stub housing within a wellhead housing.
2. Description of the Prior Art
A typical subsea wellhead assembly includes a wellhead housing mounted within a temporary guide base that is supported on the ocean floor. Large diameter conductor pipe is secured to the wellhead housing and extends downward into the earth a short distance. A wellhead is mounted inside the wellhead housing and to a permanent guide base which mounts on top of the temporary guide base. Surface casing secured to the wellhead extends a few hundred feet down into the well. The top of the wellhead is connected to pressure equipment and risers that extend to a drilling vessel at the surface. As the well is drilled deeper, a first string or casing may be set to a certain depth. Subsequently, a second string of casing may be set.
In a typical installation, the first string of casing will locate on an annular shoulder provided in the wellhead. Cement is pumped down the first string to flow back up between the casing string and the earth formation. Then seals are actuated to seal the first string to the wellhead. The second string locates on top of the first string. As a result, downward pressure due to a pressure test on the second string bears against the first string, which transmits the force to the shoulder. This places great stress on the shoulder in the wellhead. It would be desirable to have means for independently securing the casing hangers to the wellhead, so that one casing hanger did not impose an axial force on the other casing hanger.
There are other instances, as well, where it will be useful to secure one tubular member within another tubular member so as to resist axial forces. One such instance is locating a stub housing within a wellhead housing. A stub housing is a tubular member that may be subjected to high internal forces that would exert an upward force on the stub housing, tending to force it out of the wellhead housing.